Of the more than 1.11 trillion online display ads U.S. Internet users viewed in the first quarter, nearly one in three, or 31.2%, were delivered on Facebook, according to web measurement firm comScore Inc.

The leading social network led all web sites that displayed ads with 346.46 billion impressions, a 96.5% jump from the 176.31 billion impressions it delivered in the same period a year ago. Yahoo Inc. sites came in second with 112.51 billion impressions, followed by Microsoft Corp. (53.59 billion), AOL Inc. (33.45 billion) and Google Inc. sites (27.99 billion).

Facebook has increasingly focused on online advertising, rolling out offerings such as Sponsored Stories, an ad service that lets companies place their logos alongside content from Facebook posts that involve the company. Consumers can click the ads, which appear on the right-hand column of news feed pages in a box labeled Sponsored Story, to visit advertisers’ Facebook pages.

Online display advertising is thriving, says Jeff Hackett, comScore executive vice president.

 “We are now seeing more than one trillion display ads delivered every single quarter and nearly 300 individual advertisers spending at least $1 million a quarter on display, numbers which underscore just how large and vibrant the online medium has become,” he says. “And it’s not just about the volume but about the quality of the advertising experience that can be delivered as we see continued investment in compelling, high-quality creative that helps cultivate long-term brand equity.”

Telecommunications giant AT&T Inc. was the leading online display advertiser, having garnered more than 19.47 billion impressions during the quarter, followed by Experian Interactive (16.64 billion) and Scottrade Inc. (11.23 billion). Ads for Netflix Inc., No. 13 on Internet Retailer’s Top 500 Guide, captured 8.79 billion impressions and Groupon Inc. captured 7.68 billion.